Make "context" parameter really optional.

Reported by Timothee Lacroix.

slapos/recipe/template/jinja2_template.py

@@ -75,7 +75,7 @@ class Recipe(object):
         self.context = context = {}
         if _buildout_safe_dumps is not None:
             context[DUMPS_KEY] = _buildout_safe_dumps
-        for line in options.get('context').splitlines(False):
+        for line in options.get('context', '').splitlines(False):
             if not line:
                 continue
             expression_type, variable_name, expression = line.split(None, 2)